In material processing by means of laser radiation, the laser beam's exactness of positioning usually determines the precision achieved in processing. If the laser beam is focused into a processing volume, exact three-dimensional positioning is required. If the object to be machined has a deformable surface, it is usually indispensable in high-precision processing to know the surface shape or to keep any deviation of the surface shape from a predefined shape as small as possible. The above-mentioned contact element serves such purposes, because it imparts a desired surface curvature to the surface of the object to be processed.
In materials having only minor linear optical absorption within the spectral range of the processing laser radiation advantage is usually taken of non-linear interactions between the laser radiation and the material, most often in the form of an optical breakthrough being generated in the focus of the laser beam. Since the processing effect thus only takes place in the laser beam focus, exact three-dimensional orientation of the focal point is indispensable. Thus, the machining of larger areas also requires an exact depth position of the focal location in addition to two-dimensional deflection of the laser beam. Due to the contact element, known optical relationships, in particular relationships of diffraction, with the object are present. In addition, the contact element also fixes the object in a defined position relative to the processing device.
A typical application of such a contact element is the ophthalmic surgery method known as fs-LASIK, wherein a laser beam is focused in the cornea to a focal point with an order of magnitude of a few micrometers. In the focus, a plasma then forms which suddenly evaporates and disrupts the surrounding tissue. This type of interaction between laser light and tissue is referred to as photodisruption. Since photodisruption ideally remains limited to a microscopically small zone of interaction, precise surgical cuts can be performed within the eye. Local separation of corneal tissue is effected. A suitable sequential arrangement of the local separation zones thus generated realizes macroscopic cuts and isolates a defined partial volume of the cornea. Removal of said partial volume then achieves a desired change in refraction of the cornea, thus enabling correction of an eyesight defect.
Exact positioning of the laser beam is indispensable to carry out the method. A randomly involuntary movement of the human eye during treatment is problematic. Mechanical fixation of the eye or optical feedback with respect to the eye movement is required in order to minimize this factor of influence. This is why the above-mentioned contact element is used having a double function: Not only does it ensure the required optical properties when passing the laser beam into the cornea, but it also fixes the eye, preferably with regard to several degrees of freedom, particularly preferably with regard to all possible degrees of freedom. Movements of the eye relative to the laser processing device are thus prevented.
U.S. Pat. No. 6,342,053 proposes to fix the eye by means of a vacuum ring. A coupling medium in front of the eye significantly reduces the difference in refractive index with respect to the cornea. The use of this coupling medium facilitates optical correction of the system. Since said medium has a refractive index of >1, the beam deflection at the boundary surface is further reduced and any aberrations generated at this surface are reduced. In case the refractive indices of the contact glass and of the cornea are identical, no boundary surface exists from a geometrical/optical point of view.
A different concept is described in U.S. Pat. No. 5,549,632. The corneal curvature is nullified by means of a plane-parallel plate or is deformed by a concave or convex surface. This is effected by pressure on the eye. The eye is fixed, and the focused laser bundle is not affected negatively by excessively oblique incidence on a boundary surface. The pressure on the cornea inevitably leads to an increase in the internal pressure of the eye. From a medical point of view, this increase bears risks. Further, “flattening” the cornea in order to achieve a planar geometry is inconvenient for the patient.
High field strengths are a prerequisite for the process of photodisruption; these are realized by small focus diameters and short laser pulses. Small focus diameters can be achieved only with great apertures. Moreover, fields of treatment having a diameter of more than 8 mm are of interest. The geometry of the cornea results in a curved image field. No systems are known to reach an aperture of more than 0.3 with such fields. Therefore, the prior art either is either limited to smaller processing fields or works with planar geometry.
It is an object of the invention to improve a coupling element or a laser processing device of the above-mentioned type such that larger processing fields are also possible without planar geometries.